How to Design a Low Voltage Outdoor Lighting System

Low voltage
(12 volt) systems are popular for landscape
lighting for many good reasons. Low voltage systems are
perfect for creative highlighting of trees, gardens, artwork and other
landscape features. Low voltage fixtures
are easy to install. Low
voltage cable can be buried in a shallow trench, run through conduit
or buried underground. A low voltage system is flexible. You can
easily move fixtures to accomodate plant growth or to change lighting
effects. Low voltage systems are safe and energy efficient. Transformers
reduce standard voltage from 120 volts to a safe 12 volts. There is no
risk of electrical shock to children or animals if the cable is accidentally
cut. Lamps use less wattage and may not have the brightness of 120 volt,
higher wattage lamps.

Deciding
what you want to light and which techniques to use.

The first
step in designing any lighting system is to determine what you
want the light to do for you. Walk around your property, look out your
windows and doors, and think about the kind of mood you would like to
create. Look for possible focal points for dramatic accent lighting or
plan for soft but safe pathway lighting. Create romantic shadows with
subtle moonlighting or make entertainment areas come alive with bright
party lighting. Next; review the techniques of accent
lighting. Select the technique you feel best achieves the mood you
with to create. Some light will "spill" from the accent areas. Work outward
from these accent points to fill in areas with path or spead lighting.
Balance and subtlety are key tools in path and spread lighting. Try out
different techniques, and have fun discovering the beauty of your yard
at night. Be sure that when you place your fixtures they will not interfere
with lawnmowers, traffic or be a source of irritating glare inside your
house or to your neighbors.

Using
Photometrics

Once you
have developed your lighting plan, select the style of fixture and lamps
that best fit your needs. Use the photometric
charts and the lamp guide.PDF
to select lamp types and to determine light output and beam spreads. Remember,
sometimes less is more...you do not need high levels of illumination coming
from individual fixtures. Instead, use several fixtures at lower wattages
for better effect.

Low voltage
systems require the use of transformer to reduce standard 120 volt
power from your home to 12 volts. To determine the transformer size you
will need, add up the wattages of all lamps you plan to use. Select a
transformer that matches as closely as possible the total lamp wattage.
For example: if you have 11 fixtures all rated at 24.4 watts-you will
need a 300 watt (VA) transformer. (11 x 24.4 = 268.4 watts). Generally,
the total lamp load should not be less that one-third the transformer's
wattage rating, nor exceed its maximum watttage capacity. If your total
wattage is too high, either divide the load between two transformers or
use a more powerful transformer. Refer to the transformers section.

Planning
Low Voltage Layouts

These diagrams
show some of the most common low voltage cable layouts. Your choice of
layout can help minimize voltage drop.

1. Straight
run installation
Fixtures run in sequence directly from the transformer.

2. Loop
installation
Helps reduce voltage drop and produces a more uniform light output. It
is important that you connect the same wire leads to the proper transformer
terminals by noting the ridge or marking on one side of the cable.

3. Split
load installation
Run up to the recommended maximum cable length in two or more directions
from the transformer.

4. "I"
Installation
Allows more equal distribution of power to the center of a run, or to
a run some distance away. Cable running from transformer must be of a
heavier gauge (8 or 10 gauge).

How to
minimize Voltage Drop

The closer
your lamps are placed to the transformer, the higher their voltage
(and wattage) readings will be. Those farthest away will have lower voltages.
If a cable run is too long, or it too many lights are being powered by
a single transformer, noticable voltage drop may occur. Voltage drop causes
the lights farthest from the transformer to become dim. Voltage drop can
be minimized in several different ways:

use heavier
gauge cable 8 gauge K-15503 BK 10 gauge K-15504 BK

use a
higher rated transformer

multiple
taps 12-13-14

use multiple
transformers

shorten
cable lengths

reduce
individual fixture wattages

reduce
the total number of fixtures on a run Voltage drop can actually work
to your advantage if differences in lamp brightness levels are not objectionable.
Lower voltage will extend the life of a lamp, requiring less frequent
replacement.

Line
voltage (120 volt) systems are used where brilliant illumination is
required for safety or security, for lighting large areas, or when you
must light from farther away, such as uplighting large trees. In many
cases, a landscape lighting design can be developed using both line voltage
and low voltage components. The same steps used to design a low voltage
system apply when designing landscape lighting using a line voltage system.
Focus attention on the key points of interest using accent lighting. Provide
for safe access of walkways and stairs. Illuminate activity areas as needed
to maintain the mood you wish to achieve. Installation of a line voltage
system is more complicated than a low voltage system. We recommend always
working with a professional electrical contractor to make sure that your
system is installed correctly and meets local electrical codes and requirements.